1. Field of the Invention
The present invention relates to an imaging lens for reading out images, employed in an optical system of an image readout apparatus such as facsimile machine or image scanner, for example, and an image readout apparatus using the same; and, in particular, to an image readout lens for reducing or enlarging images.
2. Description of the Prior Art
Imaging lenses for image readout employed in facsimile machines, image scanners, and the like of a type adapted to form reduced or enlarged images of original images onto imaging devices such as CCD have been basically required to have a high resolution at an imaging magnification employed, a large marginal light quantity, and a small distortion. In addition, together with the recent demands for the optical apparatus as a whole to be more compact and less expensive, the lenses have also been required to have a compact size and to be made at a lower cost.
Known as an imaging lens which can correspond to such requirements are compact, four-element imaging lenses disclosed in Japanese Unexamined Patent Publication No. 1-183614 and 9-101452.
In order to achieve performances equivalent to those of a six-element imaging lens in a four-element lens configuration, the imaging lens disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 1-183614 employs an expensive glass material having a high refractive index.
Though the above-mentioned Japanese Unexamined Patent Publication No. 9-101452 includes an example using a relatively inexpensive glass material, such an example yields a brightness on the order of F/5 to 5.6, and a half angle of view .omega. on the order of 19.degree. to 20.degree.. On the other hand, examples yielding F/3.5 to 4 have a half angle of view .omega. on the order of 17.degree. to 18.degree. and employ an expensive glass material. Consequently, inexpensive imaging lenses have been desired, which are further excellent in terms of performances.
By increasing the angle of view, such an image readout lens can shorten the distance between the object and the image surface, so as to reduce the size of the apparatus. Consequently, there has been a demand for an image readout lens having a wider angle of view while maintaining high performances.